With known molded articles made of composite materials, especially of metals and nonmetallic substances, a difficulty has been encountered at the above temperature conditions, because the largely differing coefficients of thermal expansion of the various materials present in the composite must be considered and the consequences caused by these differing coefficients must be overcome.
A conventional composite body is not suitable for use at elevated temperatures over extended periods, especially when the metallic component of the composite is exposed, even if only partially, and when the operating environment causes oxidation.
It has therefore been previously attempted to protect metallic materials or composite materials containing metal components from attack by oxidation, by providing the metal component with a protective coating. This protection, however, will last only as long as the protective coating is not destroyed by thermal fatigue and/or mechanical stress. Even minute damage to the protective coating may be sufficient to induce a total failure of the structural member within a very short span of time. This holds true especially when the operating temperature is substantially higher than the temperature at which the material, if unprotected, would inherently resist oxidation.
Fiber-reinforced carbon structural members carrying a protective silicon carbide layer or coating, e.g., have been known to survive service temperatures as high as about 1600 centigrade degrees with the aid of this film-type coating, but when the protective coating was injured, a time of a minute or less was sufficient to cause the component to burn.